Polyacetal resin composition and automobile part

ABSTRACT

A polyacetal resin composition contains a polyacetal copolymer resin (A) in an amount of 100 parts by mass, the polyacetal copolymer resin having a hemiformal terminal group amount of 0.8 mmol/kg or less, a hindered phenol-based antioxidant (B) in an amount of 0.2 to 2.0 parts by mass, at least one of magnesium oxide or zinc oxide (C) in an amount of more than 2.0 parts by mass and 20 parts by mass or less, a carbon-based conductive additive (D) in an amount of 0.3 to 2.5 parts by mass, and polyalkylene glycol (E) in an amount of 0.5 to 3.0 parts by mass, wherein the carbon-based conductive additive (D) is one selected from a group consisting of only a carbon nano-structure (D1) and a combination of the carbon nano-structure (D1) and carbon black (D2) having a BET specific surface area of 300 m 2 /g or more.

TECHNICAL FIELD

The present invention relates to a polyacetal resin composition, anautomobile part obtained by molding the polyacetal resin composition,and a method for improving resistance to an acid component whileproviding conductivity to a polyacetal resin molded article.

BACKGROUND ART

A polyacetal resin or a polyacetal copolymer (hereinafter, also referredto as a “POM resin”) is excellent in various physical and mechanicalproperties, chemical resistance, and slidability, and hence is used asengineering plastic in various fields. For example, the POM resin isexcellent in resistance to hydrocarbon fuels such as gasoline, and henceis used as a flange or a case-like molded article in a periphery of afuel pump of an automobile.

Meanwhile, in certain regional areas, a cleaner having strong acidity(pH=approximately 1) is often used as a cleaning agent for an automobilewheel. Further, when a droplet of the cleaner adheres to an exposedportion of the above-mentioned flange at the time of using the cleaner,the flange surface may be degraded (decomposed) due to poor resistanceof the POM resin to an acid component. As a result, the flange may crackfrom the degraded portion as a starting point.

In view of this, for the purpose of improving resistance of the POMresin to an acid component, the applicant of the present application hasproposed a POM resin composition to which a large amount of magnesiumoxide or the like being a base is added (see Patent Literature 1 andPatent Literature 2).

Meanwhile, for the purpose of preventing ignition to fuels due to staticelectricity, in a case of a molded article used in the periphery of theabove-mentioned fuel pump, it is required to provide conductivity to themolded article and prevent electrification. As a measure to provideconductivity to the POM resin, it has been known to add a conductivefiller such as carbon black and carbon fibers (see Patent Literature 3and Patent Literature 4).

CITATION LIST Patent Literature

[Patent Literature 1] Japanese Patent No. 6386124

[Patent Literature 2] Japanese Patent No. 6691171

[Patent Literature 3] Japanese Examined Patent Publication No. 07-002891

[Patent Literature 4] Japanese Translation of PCT InternationalApplication Publication No. 2004-526596

SUMMARY OF INVENTION Technical Problem

However, when the magnesium oxide or the like is added in order toprovide resistance to an acid component, and the conductive filler suchas carbon black is further added in order to exert an anti-staticeffect, toughness of the POM resin composition is significantlydegraded, which causes a problem. In other words, both an anti-staticeffect and resistance to an acid component are to be exerted at the sametime in the POM resin composition, which leads to significantdegradation of toughness.

The present invention has been made in view of the above-mentionedproblem in the related art, and has an object to provide a POM resincomposition and an automobile part to which resistance to an acidcomponent and an anti-static effect are provided without significantdegradation of toughness, and a method for providing an anti-staticeffect to a POM resin molded article and improving resistance to an acidcomponent.

Solution to Problem

In order to solve the above mentioned problem, an aspect of the presentinvention is as below.

(1) A polyacetal resin composition, containing:

-   -   a polyacetal copolymer resin (A) in an amount of 100 parts by        mass, the polyacetal copolymer resin having a hemiformal        terminal group amount of 0.8 mmol/kg or less;    -   a hindered phenol-based antioxidant (B) in an amount of 0.2 to        2.0 parts by mass;    -   at least one of magnesium oxide or zinc oxide (C) in an amount        of more than 2.0 parts by mass and 20 parts by mass or less;    -   a carbon-based conductive additive (D) in an amount of 0.3 to        2.5 parts by mass; and    -   polyalkylene glycol (E) in an amount of 0.5 to 3.0 parts by        mass, wherein    -   the carbon-based conductive additive (D) is one selected from a        group consisting of only a carbon nano-structure (D1) and a        combination of the carbon nano-structure (D1) and carbon black        (D2) having a BET specific surface area of 300 m²/g or more.

(2) The polyacetal resin composition according to the item (1), whereina mass ratio ((D2)/(D1)) of the carbon black (D2) to the carbonnano-structure (D1) is 10 or less.

(3) The polyacetal resin composition according to the item (1) or (2),wherein the magnesium oxide has a BET specific surface area of 100 m²/gor more, and has an average particle diameter of 1.5 μm or less.

(4) An automobile part comprising a molded article of the polyacetalresin composition according to any one of the items (1) to (3).

(5) The automobile part according to the item (4), being used under acontact environment with an acidic cleaning agent.

(6) A method for providing an anti-static effect to a polyacetal resinmolded article and improving resistance to an acid component, the methodusing the polyacetal resin composition according to any one of the items(1) to (3).

(7) The method according to the item (6), wherein the acid component isderived from an acidic cleaning agent.

Advantageous Effects of Invention

According to the present invention, the POM resin composition and theautomobile part to which resistance to an acid component and ananti-static effect are provided without significant degradation oftoughness, and the method for providing an anti-static effect to the POMresin molded article and improving resistance to an acid component canbe provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically illustrating a carbon nano-structureunder states (A) before melting/kneading, (B) directly aftermelting/kneading is started, and (C) after melting/kneading.

FIG. 2 is (A) a top view and (B) a back view of a test piece used formearing a surface resistivity and a volume resistivity in Examples.

DESCRIPTION OF EMBODIMENTS

<Polyacetal Resin Composition>

A POM resin composition of the present embodiment contains a polyacetalcopolymer resin (A) in an amount of 100 parts by mass, the polyacetalcopolymer resin having a hemiformal terminal group amount of 0.8 mmol/kgor less, a hindered phenol-based antioxidant (B) in an amount of 0.2 to2.0 parts by mass, at least one of magnesium oxide or zinc oxide (C) inan amount of more than 2.0 parts by mass and 20 parts by mass or less, acarbon-based conductive additive (D) in an amount of 0.3 to 2.5 parts bymass, and polyalkylene glycol (E) in an amount of 0.5 to 3.0 parts bymass. Further, the carbon-based conductive additive (D) is one selectedfrom a group consisting of only a carbon nano-structure (D1) and acombination of the carbon nano-structure (D1) and carbon black (D2)having a BET specific surface area of 300 m²/g or more.

In the POM resin composition of the present embodiment, at least one ofthe magnesium oxide or the zinc oxide (C) is mixed by a predeterminedamount in the POM resin, and thus resistance to an acid component can beprovided. Further, the carbon-based conductive additive (D) is mixed.Thus, conductivity can be provided, and an anti-static effect can beexerted. Here, in the related art, carbon black or the like is added toexert an anti-static effect, which leads to significant degradation oftoughness in combination with the magnesium oxide or the like. However,in the present embodiment, the carbon-based conductive additive (D)provides conductivity, and hence significant degradation of toughnesscan be suppressed. The mechanism thereof is described later.

Each of the components of the POM resin composition of the presentembodiment is described below.

[Polyacetal Copolymer (A)]

In the present embodiment, the polyacetal copolymer (A) having specificterminal characteristics is used as a base resin. The polyacetalcopolymer is a resin having an oxymethylene group (—OCH₂—) as a mainstructural unit and another comonomer unit in addition to theoxymethylene unit. Further, in general, the polyacetal copolymer ismanufactured by copolymerizing formaldehyde or a cyclic oligomer offormaldehyde as a main monomer with a compound selected from a cyclicether or a cyclic formal as a comonomer. Further, in general, unstableparts at the terminal are removed and stabilized by hydrolysis.

In particular, as the main monomer, trioxane being a cyclic trimer offormaldehyde is generally used. Trioxane is generally obtained byreacting an aqueous formaldehyde solution in the presence of an acidiccatalyst, and is used after being purified by a method such asdistillation. Trioxane used for polymerization preferably contains aslittle impurities as possible, such as water, methanol, and formic acid,which is as described below.

Further, examples of the cyclic ether or the cyclic formal being thecomonomer include ethylene oxide, propylene oxide, butylene oxide,cyclohexene oxide, oxetane, tetrahydrofuran, trioxepane, 1,3-dioxane,1,3-dioxolane, propylene glycol formal, diethylene glycol formal,triethylene glycol formal, 1,4-butanediol formal, and 1,6-hexanediolformal.

Further, a compound capable of forming a branched or cross-linkedstructure may be used as the comonomer (or a termonomer), and examplesof the compound include alkyl or aryl glycidyl ethers, such as methylglycidyl ether, ethyl glycidyl ether, butyl glycidyl ether,2-ethyl-hexyl glycidyl ether, and phenyl glycidyl ether, and diglycidylethers of alkylene glycols or polyalkylene glycols, such as ethyleneglycol diglycidyl ether, triethylene glycol diglycidyl ether, andbutanediol diglycidyl ether. These comonomers may be used alone or incombination of two or more.

The polyacetal copolymer as described above may generally be obtained byadding an appropriate amount of a molecular weight regulator andperforming cationic polymerization using a cationic polymerizationcatalyst. Molecular weight regulators, cationic polymerizationcatalysts, polymerization methods, polymerization apparatuses,deactivation processes of catalysts after polymerization, terminalstabilization treatments of crude polyacetal copolymers obtained bypolymerization, and the like that may be used herein are publicly knownfrom a number of documents, and any of them may basically be used.

The molecular weight of the polyacetal copolymer (A) used in the presentembodiment is not particularly limited, and the weight average molecularweight thereof is preferably about 10,000 to about 400,000. Further, amelt mass flow rate (MFR) being an index of fluidity of the resin(measured at 190° C. under a load of 2.16 kg in accordance with ISO1133) is preferably 0.1 to 100 g/10 min, more preferably, 0.5 to 80 g%10 min.

The polyacetal copolymer (A) used in the present embodiment is requiredto have the specific terminal characteristics as described above,specifically, is required to have the hemiformal terminal group amountof 0.8 mmol/kg or less.

Here, the hemiformal terminal group is represented by —OCH₂OH, and thehemiformal terminal group amount may be obtained by ¹H-NMR measurement.With regard to the specific measurement method, reference may be made tothe method disclosed in Japanese Unexamined Patent ApplicationPublication No. 2001-11143.

When the polyacetal copolymer (A) to be used does not have theabove-mentioned terminal characteristics and exceeds the upper limitvalue, a POM resin composition in which a formaldehyde generation amountis sufficiently reduced cannot be obtained. Moreover, it is difficult tomaintain, to a low level, a generation amount of formaldehyde generateddue to repeated thermal history.

In such a case, mold deposits generated at the time of molding areexcessively increased, which hinders molding. Further, generation offormaldehyde may promote void generation in a molded article, and maycause a failure in mechanical properties.

In such a view of maintaining moldability while maintaining acidresistance, the polyacetal copolymer (A) used in the present embodimentpreferably has the hemiformal terminal group amount of 0.6 mmol/kg orless, more preferably, 0.4 mmol/kg or less. The lower limit of thehemiformal terminal group amount is not particularly limited.

The polyacetal copolymer (A) having the specific terminalcharacteristics as described above may be produced by reducingimpurities contained in the monomer and the comonomer, selecting amanufacturing process, optimizing a manufacturing condition, or thelike.

Specific examples of the method for manufacturing the polyacetalcopolymer (A) having the specific terminal characteristic according tothe present embodiment is described below, but the method is not limitedthereto.

First, it is important to reduce active impurities forming unstableterminals in the polymerization system, specifically, impurities such aswater, alcohols (for example, methanol), and acids (for example, formicacid) contained in the monomer and the comonomer.

As a matter of course, an excessively high content amount of theimpurities is not preferred for obtaining a polyacetal copolymer havinga small unstable terminal portion. Note that a chain transfer agent thatdoes not form an unstable terminal, for example, a low molecular weightlinear acetal having alkoxy groups at both terminals, such as methylal,may be contained by a freely-selected amount, and thus the molecularweight of the polyacetal copolymer may be adjusted.

Next, an amount of a catalyst that is used in a polymerization reactionis also a key factor. An excessively high catalyst amount causes adifficulty in controlling an appropriate polymerization temperature, anda decomposition reaction is dominant during the polymerization. As aresult, it is difficult to obtain a polyacetal copolymer having a smallunstable terminal portion. In contrast, an excessively low catalystamount causes reduction in polymerization reaction speed or reduction inpolymerization yield, which is not preferred.

Any publicly-known method in the related art may be adopted as thepolymerization method. A continuous mass polymerization method forobtaining a polymer in a solid powder mass form along with progressionof polymerization using a liquid monomer is industrially preferred. Apolymerization temperature is maintained preferably from 60 to 105° C.in particular, from 65 to 100° C.

When a catalyst containing boron trifluoride or a coordination compoundthereof is used, a method for adding a polymer after polymerization intoan aqueous solution containing a basic compound may be adopted as amethod for deactivating the catalyst after polymerization. However, forthe purpose of obtaining the polyacetal copolymer according to thepresent embodiment, it is preferred that the polymer obtained from thepolymerization reaction be pulverized, fragmented, and brought intocontact with a deactivation agent so as to deactivate the catalystquickly.

For example, it is desired that the polymer for catalyst deactivation bepulverized and fragmented so that 80% by mass or more, preferably, 90%by mass of the polymer have a particle diameter of 1.5 mm or less, and15% by mass or more, preferably, 20% by mass or more have a particlediameter of 0.3 mm or less.

As the basic compound for neutralizing and deactivating thepolymerization catalyst, ammonia, amines such as triethylamine,tributylamine, triethanolamine, and tributanolamine, oxides, hydroxidesand salts of alkali metals or alkaline earth metals, and otherpublicly-known catalyst deactivation agents may be used. These basiccompounds are preferably added as an aqueous solution in a concentrationof 0.001 to 0.5% by mass, particularly, 0.02 to 0.3% by mass.

Further, a preferred temperature of the aqueous solution is from 10 to80° C., particularly preferably, from 15 to 60° C. Further, aftercompletion of polymerization, it is preferred that the polymer bequickly fed into the aqueous solution for catalyst deactivation.

With reduction of impurities contained in the monomer and the comonomer,selection of the manufacturing process, and optimization of themanufacturing condition that are described above, the polyacetalcopolymer with a small unstable terminal amount can be manufactured.Further, by performing a stabilization step, the hemiformal terminalgroup amount can further be reduced.

Examples of the stabilization step include publicly-known methods suchas a method in which the polyacetal copolymer is heated to a temperatureequal to higher than a melting point thereof and is subjected toprocessing under a molten state so as to decompose and remove only anunstable portion, and a method in which the polyacetal copolymer issubjected to heating processing at a temperature equal to or higher than80° C. while maintaining a heterogeneous system in an insoluble liquidmedium so as to decompose and remove only an unstable terminal portion.

[Hindered Phenol-Based Antioxidant (B)]

Examples of the hindered phenol-based antioxidant (B) used in thepresent embodiment include 2,2′-methylenebis(4-methyl-6-t-butylphenol),hexamethylene-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane,triethyleneglycol-bis[3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionate],1,3,5-trimethyl-2,4,6-tris(3′,5′-di-t-butyl-4-hydroxy-benzyl)benzene,n-octadecyl-3-(4′-hydroxy-3′,5′-di-t-butylphenol)propionate,4,4′-methylenebis(2,6-di-t-butylphenol),4,4′-butylidenebis(6-t-butyl-3-methyl-phenol),distearyl(3,5-di-t-butyl-4-hydroxybenzyl)phosphonate,2-t-butyl-6-(3-t-butyl-5-methyl-2-hydroxybenzyl)-4-methylphenylacrylate,and3,9-bis{2-[3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionyloxy]-1,1-dimethylethyl}-2,4,8,10-tetraoxaspiro[5,5]undecane.Among those,tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methaneand hexamethylene-bis[3-(3,5-di-t-butyl-4-hydroxyhyphenyl)propionate]are preferred.

In the present embodiment, at least one or two or more types selectedfrom those antioxidants may be used.

In the present embodiment, the amount of the hindered phenol-basedantioxidant (B) is 0.2 to 2.0 parts by mass, more preferably, 0.2 to 1.5parts by mass, with respect to 100 parts by mass of the POM resin (A).

[Magnesium Oxide, Zinc Oxide (C)]

At least one of the magnesium oxide or the zinc oxide (hereinafter, alsoreferred to as “component (C)”) is mixed in the POM resin composition ofthe present embodiment. The component (C) used in the present embodimentis excellent in balance between improvement of resistance to a cleaningagent (resistance to an acid component (hereinafter, also referred to as“acid resistance”)) and performance such as mechanical properties andmoldability, which is preferred.

The magnesium oxide preferably has a BET specific surface area of 100m²/g or more and an average particle diameter of 1.5 μm or less. Whenthese conditions are satisfied, acid resistance can be obtained whilesuppressing degradation of toughness. The BET specific surface area ofthe magnesium oxide is preferably 100 to 500 m²/g, more preferably, 120to 300 m²/g. Further, the average particle diameter of the magnesiumoxide is preferably 0.2 to 1.3 μm, more preferably, 0.3 to 1.0 μm. Theaverage particle diameter is determined as a particle diametercorresponding to 50% of the integrated value in a particle sizedistribution (volume-based) measured by a laser diffraction/scatteringmethod.

In the present embodiment, the amount of the component (C) is preferablymore than 2.0 parts by mass and 20 parts by mass or less, morepreferably, 4.0 parts by mass or more and 15 parts by mass or less, withrespect to 100 parts by mass of the POM resin (A). When the amountthereof exceeds 2.0 parts by mass, the component (C) is particularlyexcellent in acid resistance. When the amount thereof falls within 20parts by mass, the component (C) can stably be produced. When the amountthereof falls within 10 parts by mass, the component (C) is particularlyexcellent in balance of mechanical characteristics. In the related art,an increase of the component (C) may promote decomposition of anunstable terminal in a POM resin. The POM resin (A) of the presentembodiment can suppress the decomposition thereof. Therefore, there hassuccessfully been found characteristics in improvement of acidresistance by increasing an amount of the component (C).

[Carbon-Based Conductive Additive (D)]

In the POM resin composition of the present embodiment, the carbon-basedconductive additive (D) is mixed by a predetermined amount with respectto the POM resin (A). The carbon-based conductive additive (D) is oneselected from a group consisting of only the carbon nano-structure (D1)(hereinafter, also referred to as “CNS”) and a combination of the carbonnano-structure (D1) and the carbon black (D2) having a BET specificsurface area of 300 m²/g or more. Further, conductivity is provided, andan anti-static effect is exerted by adding the carbon-based conductiveadditive (D) to the POM resin composition. When the carbon black isadded alone, toughness of a molded article thus obtained is degraded.However, degradation of toughness can be suppressed by adding thecarbon-based conductive additive (D).

Each of the carbon nano-structure (D1) and the carbon black (D2) havinga BET specific surface area of 300 m²/g or more is described below.

(Carbon Nano-Structure (CNS) (D1))

The CNS used in the present embodiment is a structure body including aplurality of carbon nanotubes in a coupled state in which a carbonnanotube is coupled to another carbon nanotube throughbranching/crosslinking or a bridging structure. Details of the CNS ofthis kind are described in US Unexamined Patent Application PublicationNo. 2013-0071565, U.S. Pat. Nos. 9,133,031, 9,447,259, and 9,111,658.

With reference to the drawing, modes of the CNS are described. FIG. 1schematically illustrates the CNS used in the present embodiment: thepart (A) illustrates a state before melting/kneading with a POM resin;the part (B) illustrates a state directly after melting/kneading isstarted; and the part (C) illustrates a state after melting/kneading. Asillustrated in FIG. 1(A), a CNS 10 before melting/kneading has astructure body in which a large number of branched carbon nanotubes 12are intertwined and coupled to each other.

Further, the CNS 10 is fed into a POM resin 20, and is subjected tomelting/kneading. Then, as illustrated in FIG. 1(B), the CNS 10 isdivided into a number of pieces. As melting/kneading progresses, the CNS10 is further divided. Then as illustrated in FIG. 1(C), each one of thecarbon nanotubes 12 is in a contact state with another one thereof via acontact point 14. In other words, in the state of FIG. 1(C), there isestablished a state in which the large number of carbon nanotubes 12contact with one another in a wide range in the POM resin, and aconductive path is formed. Thus, conductivity is exerted. Further, it isconsidered that degradation of toughness can be suppressed because thecarbon nanotubes 12 are intertwined in a random manner to form athree-dimensional network structure.

In order to obtain the CNS in the state illustrated in FIG. 1(C), theCNS illustrated in FIG. 1(A) preferably has a predetermined flake-likeshape. The CNS having a flake-like shape, which is illustrated in FIG.1(A), contains the plurality of carbon nanotubes. The carbon nanotubesare branched and bridge-coupled, and mutually have a commonly-sharedwall. In this case, not all the plurality of carbon nanotubes arebranched and bridge-coupled, and mutually have a commonly-shared wall,and it is only required that the plurality of carbon nanotubes as awhole have at least one of these structural characteristics. Further,the CNS having a flake-like shape as described above is used, and thusthe state illustrated in FIG. 1(C) is established throughmelting/kneading.

The CNS having a flake-like shape as described above is obtained bygrowing a CNS on a growth base material such as a fiber material andextracting the grown CNS from the growth base material. For the CNSgrowth process, the growth base material such as fibers, tow, thread,woven fabric, non-woven fabric, a sheet, a tape, and a belt may be used.In other words, the growth base material may be a fiber material havingsuch a dimension that the fiber material can be spooled, and the CNS cancontinuously be formed while transporting the growth base material.

More specifically, a catalyst is applied onto the growth base materialso as to grow the CNS by a pore CVD process. Further, the growth basematerial on which the CNS is formed may be preserved and wound up so asto extract the CNS afterwards.

A catalyst containing a plurality of transition metal nanoparticles ispreferably used for growing the CNS on the growth base material. Forexample, application of the catalyst on the growth base material may beperformed by particle adsorption such as direct catalyst applicationthrough vapor deposition of precursor in a liquid or colloidal state. Asthe transition metal nano-particle catalyst, d-block transition metal ord-block transition metal salt is contained. The transition metal saltmay be applied onto the growth base material without thermal treatment.Alternatively, the transition metal salt may be subjected to thermaltreatment, and may be transformed into zero-valent transition metal onthe growth base material.

The CNS contains the carbon nano-tubes in a network having a complexstructural form. It is considered that the complex structural form isderived from formation of the CNS on the growth base material under agrowth condition for generating the carbon nanotubes at a rapid growthspeed, approximately, several micro meters per second.

For synthesizing carbon nanotubes on a fiber material, varioustechniques for forming the carbon nanotubes may be adopted, includingthe contents disclosed in US Unexamined Patent Application PublicationNo. 2004-0245088. The CNS grown on the fibers may be formed by atechnique such as microcavity, a thermal and plasma enhancement CVDtechnique, laser ablation, arc discharge, and high-pressure carbonmonoxide (HiPCO). Acetylene gas may be ionized to generatelow-temperature carbon plasma for synthesizing carbon nanotubes. In thisstate, the plasma is oriented toward the fiber material having thecatalyst. In this manner, for synthesizing the CNS on the fibermaterial, the two conditions including (a) formation of the carbonplasma and (b) orientation of the carbon plasma toward the catalystarranged on the fiber material are preferably provided. The diameter ofthe growing carbon nanotube is defined by a size of a catalyst forcarbon nanotube formation. Further, the fiber material subjected tosizing is heated to a temperature of approximately 550 to 800° C., andthus synthesis of the CNS is facilitated. In order to initiate growth ofthe carbon nanotubes, two types of gas are caused to flow into areactor. In other words, the two types of gas include process gas suchas argon, helium, and nitrogen, and carbon-containing gas such asacetylene, ethylene, ethanol, and methane. Further, the carbon nanotubesare grown at the position of the catalyst for carbon nanotube formation.

The CNS used in the present embodiment may be a commercial product. Forexample, ATHLOS 200 or ATHLOS 100 produced by CABOT Corporation may beused.

(Carbon Black (D2) Having BET Specific Surface Area of 300 m²/g or More)

In the present embodiment, among various types of carbon black, thecarbon black having a BET specific surface area of 300 m²/g or more isused. However, the carbon black is not used along, but is used incombination with the CNS. The POM resin composition in which the carbonblack is mixed has high conductivity, and hence conductivity can bemaintained even in combination with the CNS. In contrast, a POM resincomposition in which carbon black having a BET specific surface arealess than 300 m²/g is mixed has low conductivity, and it is required toincrease the amount in order to secure sufficient conductivity. As aresult, degradation of toughness cannot be suppressed. The BET specificsurface area is preferably 310 m²/g or more, more preferably, 350 m²/gor more, and the upper limit thereof is, but not particularly limitedto, approximately 2,000 m²/g.

Note that the BET specific surface area may be measured in accordancewith ASTI D4820.

Examples of the specific carbon black as described above includeKETJENBLACK EC300J (BET specific surface area: 800 m²/g), KETJENBLACKEC600JD (BET specific surface area: 1,270 m²/g), and LIONITE EC200L (BETspecific surface area: 377 m²/g) that are produced by Lion Corporation.

In the POM resin composition of the present embodiment, the carbon-basedconductive additive (D) in an amount of 0.3 to 2.5 parts by mass ismixed with respect to 100 parts by mass of the POM resin. When theamount thereof is less than 0.3 parts by mass, the carbon-basedconductive additive (D) is inferior in conductivity. When the amountthereof exceeds 2.5 parts by mass, toughness is degraded. The amount ofthe CNS is preferably 0.5 to 2.0 parts by mass, more preferably, 0.6 to1.8 parts by mass, and further preferably, 0.8 to 1.5 parts by mass.

Further, when the CNS (D1) and the carbon black (D2) are used incombination, a mass ratio ((D2)/(D1)) of the carbon black (D2) to theCNS (D1) is preferably 10 or less, more preferably, more than 0 and 5 orless. When the mass ratio is 10 or less, balance between conductivityand toughness can be secured. Further, as the value of D2/D1 is closerto 0, the CNS is mixed by an excessively high amount. However, the CNSmay be mixed by an excessively high amount in this manner. However, inconsideration of the fact that the CNS is expensive, the lower limit ofthe value of D2/D1 is preferably 0.1 in view of cost efficiency.

[Polyalkylene Glycol (E)]

In the POM resin composition of the present embodiment, the polyalkyleneglycol (E) is mixed by a predetermined amount with respect to the POMresin (A). The type thereof is not particularly limited. In view ofcompatibility with the POM resin, the type thereof contains preferablypolyethylene glycol or polypropylene glycol, more preferably,polyethylene glycol.

The number-average molecular weight (Mn) of the polyalkylene glycol isnot particularly limited. In view of dispersibility in the polyacetalresin, the number-average molecular weight is preferably 1,000 or moreand 50,000 or less, more preferably, 5,000 or more and 30,000 or less.Note that, in the present application, the number-average molecularweight is assumed to be a number-average molecular weight in terms ofpolystyrene that is obtained by size exclusion chromatography (SEC) withtetrahydrofuran (THF) as a solvent.

In the present embodiment, the amount of the polyalkylene glycol (E) is0.5 to 3.0 parts by mass, preferably, 0.8 to 2.5 parts by mass, withrespect to 100 parts by mass of the POM resin (A). When the amount ofthe polyalkylene glycol (E) is less than 0.5 parts by mass, acidresistance and toughness are degraded. When the amount thereof exceeds3.0 parts by mass, tensile strength is reduced. The upper limit of theamount is selected in view of balance with mechanical properties of themolded article. Two types of the polyalkylene glycol may be mixed andused.

[Other Components]

The POM resin composition of the present embodiment may contain othercomponents as required. One or more kinds of publicly-known stabilizingagents may be added to the POM resin composition as long as the purposeand the effects of the POM resin composition of the present embodimentare not inhibited.

A method of producing a molded article by using the POM resincomposition of the present embodiment is not particularly limited, and apublicly-known method may be adopted. For example, a molded article maybe produced by feeding the POM resin composition of the presentembodiment into an extruding machine, subjecting the POM resincomposition to melting/kneading, forming the POM resin composition intopellets, feeding the pellets into an injection molding machine equippedwith a predetermined metal mold, and subjecting the pellets to injectionmolding.

The POM resin composition of the present embodiment described above maybe formed into an automobile part described below, or may be a moldedarticle having an anti-static function and resistance to an acidcomponent.

<Automobile Part>

An automobile part of the present embodiment is formed of a moldedarticle of the above-mentioned POM resin composition of the presentembodiment. Therefore, the automobile part of the present embodiment isprovided with resistance to an acid component and an anti-static effectwithout significant degradation of toughness. Therefore, the automobilepart is suitably used as a flange or a case-like molded article in aperiphery of a fuel pump of an automobile. In other words, theautomobile part is excellent in an anti-static effect, and hence canprevent ignition to fuels due to static electricity. The automobile partis excellent in resistance to an acid component, and hence can preventsurface degradation even when a cleaner having strong acidity(pH=approximately 1) adheres thereto. In other words, the automobilepart of the present embodiment can be used under a contact environmentwith an acidic cleaning agent.

The automobile part of the present embodiment may be obtained by usingthe above-mentioned POM resin composition and performing molding by acommon molding method such as injection molding, extrusion molding,compression molding, blow molding, vacuum molding, foam molding, androtation molding.

Even when the automobile part of the present embodiment contacts with,for example, a cleaning agent having strong acidity of pH 2 or less,degradation can be suppressed, and satisfactory molded-article surfaceappearance can be maintained.

<Method for Providing Anti-Static Effect to Polyacetal Resin MoldedArticle and Improving Resistance to Acid Component>

According to the present embodiment, the method for providing ananti-static effect to the polyacetal resin molded article and improvingresistance to an acid component uses the above-mentioned POM resincomposition of the present embodiment.

As described above, the molded article obtained by molding the POM resincomposition of the present embodiment can be provided with resistance toan acid component and an anti-static effect without significantdegradation of toughness. In other words, the POM resin composition ofthe present embodiment is used, and thus resistance to an acid componentand an anti-static effect of the POM resin composition can be exertedwithout significant degradation of toughness. As the acid component, acomponent derived from an acidic cleaning agent may can be used.

In the method of the present embodiment, each component and a preferredcontent amount thereof with respect to the POM resin and othercomponents are as described in the above-mentioned POM resin compositionof the present embodiment.

EXAMPLES

The present embodiment is further specifically described below withreference to Examples, and the present embodiment is not limited toExamples given below.

Examples 1 to 15, Comparative Examples 1 to 12

In each of Examples and Comparative Example, the respective raw materialcomponents illustrated in Table 1 and Table 2 were dry-blended. Then,the resultant was fed into a twin-screw extruding machine at a cylindertemperature of 200° C., subjected to melting/kneading, and formed intopellets. Note that, in Table 1 and Table 2, a numerical value of eachcomponent indicates parts by mass.

Further, details of the respective raw material components that wereused are described below.

Polyacetal Copolymer Resin (A) (POM Resin)

A-1: a POM resin having a hemiformal terminal group amount of 0.7mmol/kg

A-2: a POM resin having a hemiformal terminal group amount of 1.0mmol/kg

The MFRs of both A-1 and A-2 that were measured at 190° C. under a loadof 2.16 kg in accordance with ISO 1133 were 9 g/10 min.

The polyacetal copolymers A-1 and A-2 were obtained in the followingmanner.

A-1: A mixture of 96.7% by mass of trioxane and 3.3% by mass of1,3-dioxolane was continuously supplied to a continuous polymerizationmachine of a twin-screw paddle type, and 10 ppm of boron trifluoride wasadded as a catalyst to carry out polymerization. Further, the mixture oftrioxane and 1,3-dioxolane for polymerization contained 10 ppm of water,3.5 ppm of methanol, and 5 ppm of formic acid as impurities.

An aqueous solution containing triethylamine in an amount of 1,000 ppmwas immediately added to the polymer discharged from the discharge portof the polymerization machine. The resultant was subjected topulverization and stirring processing so as to deactivate the catalyst,and then was subjected to centrifugal separation and drying so as toobtain a crude polyoxymethylene copolymer.

The crude polyoxymethylene copolymer was supplied to a twin-screwextruding machine having a vent port. Then, a 0.3-percent triethylamineaqueous solution was added in an amount of 0.4% to the crudepolyoxymethylene copolymer, and the resultant was subjected tomelting/kneading at a resin temperature of approximately 220° C. Withthis, an unstable terminal portion was decomposed, and a volatilecomponent containing the decomposition product was devolatilized under areduced pressure through the vent port. The polymer extruded from thedie of the extruding machine was cooled and shredded, and thus thepolyacetal copolymer A-1 in a form of pellets from which the unstableterminal portion was removed was obtained.

A-2: A mixture of 96.7% by mass of trioxane and 3.3% by mass of1,3-dioxolane was continuously supplied to a continuous polymerizationmachine of a twin-screw paddle type, and 15 ppm of boron trifluoride wasadded as a catalyst to carry out polymerization. Further, the mixture oftrioxane and 1,3-dioxolane for polymerization contained 10 ppm of water,3.5 ppm of methanol, and 5 ppm of formic acid as impurities. After that,the polymer discharged from the discharge port of the polymerizationmachine was subjected to the processing similar to A-1 described above,and thus the polyacetal copolymer A-2 in a form of pellets was obtained.

Hindered Phenol-Based Antioxidant (B)

B-1:tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane(product name: Irganox1010, produced by BASF SE)

Magnesium Oxide or the Like (C)

C-1: magnesium oxide having a specific surface area of 135 m²/g and anaverage particle diameter of 0.9 μm (Kyowamag MF150 produced by KyowaChemical Industry Co., Ltd.)

C-2: magnesium oxide having a specific surface area of 30 m²/g and anaverage particle diameter of 0.6 μm (Kyowamag MF30 produced by KyowaChemical Industry Co., Ltd.)

C-3: magnesium oxide having a specific surface area of 155 m²/g and anaverage particle diameter of 7 μm (Kyowamag 150 produced by KyowaChemical Industry Co., Ltd.)

C-4: Zinc oxide (a BET specific surface area of 60 to 90 m²/g) (ActiveZinc Oxide AZO produced by Seido Chemical Industry Co., Ltd.)

(Measurement of Average Particle Diameter)

A particle size distribution was measured by a laserdiffraction/scattering method under the following measurement conditionsthrough use of a laser diffraction/scattering particle distributionmeasurement device LA-920 produced by HORIBA. Ltd., and thus the averageparticle diameter (50% d) corresponding to 50% of the integrated valuewas obtained.

Measurement Conditions

-   -   Circulation speed: 5    -   Laser light source: a He—Ne laser having a wavelength of 632.8        nm; 1 mW, a tungsten lamp; 50 W    -   Detector: one ring-like 75-cell silicon photodiode, twelve        silicon photodiodes    -   Dispersion medium: distilled water    -   Ultrasonic wave: present    -   Transmittance: 75 to 90%    -   Relative refractive index with water: 1.32    -   Particle size: volume-based

Carbon Nano-Structure, Carbon Black (D)

D-1: carbon nano-structure (ATHLOS200 produced by CABOT Corporation)

D-2: carbon black (KETJENBLACK EC300J having a BET specific surface areaof 800 m²/g, produced by Lion Corporation)

D-3: carbon black (LIONITE EC200L having a BET specific surface area of377 m²/g, produced by Lion Corporation)

D-4: carbon black (DENKA BLACK having a BET specific surface area of 65m²/g, produced by Denka Company Limited)

Polyalkylene Glycol (E)

E-1: polyethylene glycol (PEG6000S produced by Sanyo ChemicalIndustries, Ltd.)

<Evaluation>

The POM resin compositions produced in Examples and Comparative Examplewere used to produce multi-purpose test pieces described in ISO 294-1 byan injection molding machine (EC40 produced by Toshiba Machine Co.,Ltd.) under a condition in accordance with ISO 9988-1, 2. Themulti-purpose test pieces were used for the following evaluation items(1) to (3).

(1) Evaluation on Resistance to Acidic Cleaning Agent (Acid Resistance)

For evaluation on resistance to an acidic cleaning agent of the POMresin composition, both the ends of the multi-purpose test pieces werefixed and curved at a load strain ratio of 2.0%. Further, the acidiccleaning agent was sprayed onto the surface of the multi-purpose testpieces, and the multi-purpose test pieces after being subjected tospraying were left for four hours under a condition of 60° C. Then, themulti-purpose test pieces were left for four hours under conditions of23° C. and 50% RH. Subsequently, the acidic cleaning agent was sprayedagain, and the multi-purpose test pieces were left for 16 hours underconditions of 23° C. and 50% RH.

As the acidic cleaning agent, the following acidic cleaning agent wasused.

Cleaning agent: 1.5% of sulfuric acid, 1.5% of hydrofluoric acid, and10% of phosphoric acid

Actions of spraying an acidic cleaning agent, leaving the multi-purposetest pieces for four hours at 60° C., leaving the multi-purpose testpieces for four hours at 23° C. and 50% RH, spraying an acidic cleaningagent again, and leaving the multi-purpose test pieces for 16 hours at23° C. and 50% RH were included in one cycle. Every time the one cyclewas completed, crack generation states on surfaces of dumbbell-like testpieces were visually observed. Further, evaluation was performed basedon the cycle number from which a crack was observed, in accordance withthe following evaluation criteria. The evaluation results are shown inTable 1 and Table 2.

[Evaluation Criteria]

-   -   A: the number of cycles; 10 or more    -   B: the number of cycles: 4 to 9    -   C: the number of cycles; 3 or less

(2) Evaluation on Tensile Fracture Nominal Strain (Toughness)

The multi-purpose test pieces described above were used to measure atensile fracture nominal strain in accordance with ISO 527-1, 2, andevaluation was performed in accordance with the following evaluationcriteria. The evaluation results are shown in Table 1 and Table 2.

[Evaluation Criteria]

-   -   A: 10% or more    -   B: 6 to 9%    -   C: less than 5%

(3) Conductivity

The above-mentioned multi-purpose test pieces were used to performevaluation described below.

(Surface Resistivity, Volume Resistivity)

An outer appearance of the multi-purpose test pieces obtained asdescribed above is illustrated in FIG. 2 . FIG. 2(A) illustrates thefront surface thereof, and FIG. 2(B) illustrates the back surfacethereof. A conductive paint (Dotite D500 produced by FUJIKURA KASEI Co.,Ltd.) was applied to predetermined regions (hatched regions in FIG. 2 )on each of the surfaces of the test piece, and was dried. Then, alow-resistivity measurement device (DIGITAL MULTI METER R6450 producedby ADVANTEST Corporation) was used to measure a resistance between thearea A and the area B in FIG. 2(A) as a surface resistivity. Further, aresistance between the area C and the area D in FIG. 2 was measured as avolume resistivity. Evaluation was performed on each of the surfaceresistivity and the volume resistivity in accordance with the followingevaluation criteria. The evaluation results are shown in Table 1 andTable 2. Note that the measurement upper limit for the surfaceresistivity is 5.0×10⁹Ω/□, and the measurement upper limit for thevolume resistivity is 1.8×10¹¹ Ω·cm.

[Evaluation Criteria for Surface Resistivity]

-   -   A: 1.0-10⁴Ω/□ or less    -   B: more than 1.0×10⁴Ω/□ and 1.0×10⁹Ω/□ or less    -   C: more than 1.0×10⁹Ω/□

[Evaluation Criteria for Volume Resistivity]

-   -   A: 1.0×10⁴ Ω·cm or less    -   B: more than 1.0×10⁴ Ω·cm and 1.0×10⁹ Ω·cm or less    -   C: more than 1.0×10⁹ Ω·cm

(4) Moldability: Mold Deposits

The POM resin compositions produced in Examples and Comparative Examplewere used, and mold deposit test pieces (disk-like shape) were moldedunder the following conditions.

[Evaluation Method]

After 3,000-shot molding, the surface of the cavity member on themovable mold was visually observed, and the amount of deposits wasdetermined according to the following criteria.

-   -   A: No deposits or were observed, or a slight amount of deposits        was observed.    -   B: A large amount of deposits was observed.        -   Molding machine: FANUC ROBOSHOT S-2000i 50B (produced by            FANUC Corporation)        -   Molding condition: a cylinder temperature (° C.)

Nozzle C1 C2 C3 205 215 205 185° C.

-   -   Injection pressure: 40 (MPa)    -   Injection speed: 1.5 (m/min)    -   Mold temperature: 60 (° C.)

TABLE 1 Examples 1 2 3 4 5 6 7 8 (A) POM resin A-1 100 100 100 100 100100 100 100 A-2 — — — — — — — — (B) Hindered phenol-based antioxidantB-1 0.25 1.5 0.25 0.25 0.25 0.25 0.25 0.25 (C) Magnesium oxide C-1 5.05.0 2.0 15.0 — — — 5.0 C-2 — — — — 5.0 — — — C-3 — — — — — 5.0 — —Magnesium hydroxide C-4 — — — — — — 5.0 — (D) Carbon-based D-1 0.8 0.80.8 0.8 0.8 0.8 0.8 0.5 conductive additive D-2 — — — — — — — — D-3 — —— — — — — — D-4 — — — — — — — — (E) Polyalkylene glycol E-1 2.0 2.0 2.02.0 2.0 2.0 2.0 2.0 Acid resistance A A B A B A B A Tensile fracturenominal strain (toughness) A A A A A A A A Conductivity Surfaceresistivity [Ω/□] A A A A A A A B Volume resistivity [Ω

] A A A A A A A B Moldability A A A A A A A A Examples 9 10 11 12 13 1415 (A) POM resin A-1 100 100 100 100 100 100 100 A-2 — — — — — — — (B)Hindered phenol-based antioxidant B-1 0.25 0.25 0.25 0.25 0.25 0.25 0.25(C) Magnesium oxide C-1 5.0 5.0 5.0 5.0 5.0 5.0 5.0 C-2 — — — — — — —C-3 — — — — — — — Magnesium hydroxide C-4 — — — — — — — (D) Carbon-basedD-1 1.2 2.0 0.5 1.2 0.5 0.8 0.8 conductive additive D-2 — — 1.5 0.8 — —— D-3 — — — — 1.5 — — D-4 — — — — — — — (E) Polyalkylene glycol E-1 2.02.0 2.0 2.0 2.0 0.5 3.0 Acid resistance A A A A A B A Tensile fracturenominal strain (toughness) A B A B A A A Conductivity Surfaceresistivity [Ω/□] A A A A B A A Volume resistivity [Ω

] A A A A B A A Moldability A A A A A A A

TABLE 2 Comparative Examples 1 2 3 4 5 6 (A) POM resin A-1 100 100 100100 100 100 A-2 — — — — — — (B) Hindered phenol-based antioxidant B-10.25 0.25 0.25 0.25 0.25 0.25 (C) Magnesium oxide C-1 — 5.0 5.0 — — 1.5C-2 — — — — — — C-3 — — — — — — Magnesium hydroxide C-4 — — — — — — (D)Carbon-based conductive additive D-1 — — — 0.8 0.8 0.8 D-2 — — — — — —D-3 — — — — — — D-4 — — — — — — (E) Polyalkylene glycol E-1 — — 2.0 —2.0 2.0 Acid resistance C B A C C C Tensile fracture nominal strain(toughness) A A A A A A Conductivity Surface resistivity [Ω/□] C C C B BA Volume resistivity [Ω

] C C C B B B Moldability A A A A A A Comparative Examples 7 8 9 10 1112 (A) POM resin A-1 100 100 100 100 100 — A-2 — — — — — 100 (B)Hindered phenol-based antioxidant B-1 0.25 0.25 0.25 0.25 0.25 0.25 (C)Magnesium oxide C-1 25.0 5.0 5.0 5.0 5.0 5.0 C-2 — — — — — — C-3 — — — —— — Magnesium hydroxide C-4 — — — — — — (D) Carbon-based conductiveadditive D-1 0.8 0.3 3.0 0.3 0.5 0.8 D-2 — — — 2.0 — — D-3 — — — — — —D-4 — — — — 1.5 — (E) Polyalkylene glycol E-1 2.0 2.0 2.0 2.0 2.0 2.0Acid resistance A A A A A A Tensile fracture nominal strain (toughness)C A C B A A Conductivity Surface resistivity [Ω/□] A C A B C A Volumeresistivity [Ω

] A C A B C A Moldability A A A A A B

From Table 1 and Table 2, it can be understood that satisfactory resultswere obtained for all the evaluation items in each of Examples 1 to 15.In contrast, in Comparative Examples 1 to 12, satisfactory results couldnot obtained for all the evaluation items at the same time.

Comparative Example 1 was different from Example 1 in that thecomponents (C) to (E) were not mixed, and was inferior in acidresistance and conductivity. Comparative Example 2 was different fromExample 1 in that the components (D) and (E) were not mixed, and wasinferior in conductivity. Comparative Example 3 was different fromExample 1 in that the component (D) was not mixed, and was inferior inconductivity. Comparative Example 4 was different from Example 1 in thatthe components (C) and (E) were not mixed, and was inferior in acidresistance. Comparative Examples 5 and 6 were different from Example 1in that the component (C) was not mixed and was excessivelyinsufficient, respectively, and were both inferior in acid resistance.Comparative Example 7 was different from Example 1 in that the component(C) was excessively mixed, and was inferior in acid resistance.Comparative Example 8 was different from Example 1 in that the component(D) was excessively insufficient, and was inferior in conductivity.Comparative Example 9 was different from Example 1 in that the component(D) was excessively mixed, and was inferior in toughness. ComparativeExample 10 was different from Examples 1 and 3 in that the carbon blackhaving a BET specific surface area of less than 300 m²/g was used as thecarbon black being the component (D), and was inferior in conductivity.Comparative Example 11 was different from Example 1 in that the POMresin had an excessive hemiformal terminal group amount, and wasinferior in moldability.

1. A polyacetal resin composition, comprising: a polyacetal copolymerresin (A) in an amount of 100 parts by mass, the polyacetal copolymerresin having a hemiformal terminal group amount of 0.8 mmol/kg or less;a hindered phenol-based antioxidant (B) in an amount of 0.2 to 2.0 partsby mass; at least one of magnesium oxide or zinc oxide (C) in an amountof more than 2.0 parts by mass and 20 parts by mass or less; acarbon-based conductive additive (D) in an amount of 0.5 to 2.0 parts bymass; and polyalkylene glycol (E) in an amount of 0.5 to 3.0 parts bymass, wherein the carbon-based conductive additive (D) is one selectedfrom a group consisting of only a carbon nano-structure (D1) which is astructure body including a plurality of carbon nanotubes in a coupledstate in which a carbon nanotube is coupled to another carbon nanotubethrough branching/coupling or a bridging structure and a combination ofthe carbon nano-structure (D1) and carbon black (D2) having a BETspecific surface area of 300 m²/g or more, and in case that thecarbon-based conductive additive (D) is the combination of the carbonnano-structure (D1) and carbon black (D2) having a BET specific surfacearea of 300 m2/g or more, the carbon nano-structure (D1) is included 0.5parts by mass with respect to the 100 parts by mass of the polyacetalcopolymer resin (A).
 2. The polyacetal resin composition according toclaim 1, wherein a mass ratio ((D2)/(D1)) of the carbon black (D2) tothe carbon nano-structure (D1) is 3 or less.
 3. The polyacetal resincomposition according to claim 1, wherein the magnesium oxide has a BETspecific surface area of 100 m²/g or more, and has an average particlediameter of 1.5 μm or less.
 4. An automobile part comprising a moldedarticle of the polyacetal resin composition according to claim
 1. 5. Theautomobile part according to claim 4, being used under a contactenvironment with an acidic cleaning agent.
 6. A method for providing ananti-static effect to a polyacetal resin molded article and improvingresistance to an acid component, the method using the polyacetal resincomposition according to claim
 1. 7. The method according to claim 6,wherein the acid component is derived from an acidic cleaning agent.